In optical communication systems, messages are transmitted by carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems.
One preferred means for switching or guiding waves of optical frequencies from one point to another is by an optical waveguide. The operation of an optical waveguide is based on the fact that when a light-transmissive medium is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
A wide variety of optical devices can be made which incorporate a light guiding structure as the light transmissive elements. Illustrative of such devices are planar optical slab waveguides, channel optical waveguides, rib waveguides, optical couplers, optical splitters, optical switches, optical filters, variable attenuators, micro-optical elements and the like. These devices are described in more detail in U.S. Pat. Nos. 4,609,252, 4,877,717, 5,136,672, 5,136,682, 5,481,385, 5,462,700, 5,396,350, 5,428,468, 5,850,498, and U.S. patent application Ser. No. 08/838,344 filed Apr. 8, 1997, the disclosures of which are all incorporated herein by reference.
It is known in the art to make optical waveguides and other optical interconnect devices from organic polymeric materials. Whereas single mode optical devices made from planar glass are relatively unaffected by temperature, devices made from organic polymers show a far greater variation with temperature because the refractive index changes much faster with temperature in polymeric materials than in glass. This property can be exploited to make active, thermally tunable or controllable devices incorporating light transmissive elements made from organic polymers. One type of thermally tunable devices is a directional coupler activated by a thermo-optic effect. The thermo-optic effect is a change in the index of refraction of the optical element that is induced by heat. Thermo-optic effect devices help to provide less costly routers when the activation speed of a coupler state is not too high, i.e., when the activation speed is in the range of milliseconds.
Unfortunately, most polymeric materials contain carbon-to-hydrogen chemical bonds which absorb strongly at the 1550 nm wavelength that is commonly used in telecommunication applications. It has long been known that fluoropolymers, for example, have significantly reduced absorption at 1550 nm. While planar waveguides made from fluorinated polyimide and deuterated polyfluoromethacrylate have achieved single mode losses of as little as 0.10 db/cm at 1300 nm, it is relatively difficult to make optical devices from these materials. Specifically, the photolithographic process by which they have been made includes a reactive ion etching step. Fluorinated polyimide and deuterated polyfluoromethacrylate also have higher losses at 1550 nm, typically on the order of 0.6 dB/cm.
Photopolymers have been of particular interest for optical interconnect applications because they can be patterned using standard photolithographic techniques. As is well known, photolithography involves patternwise exposure of a light-sensitive polymeric layer deposited on a chosen substrate followed by development of the pattern. Development may be accomplished, for example, by removal of the unexposed portion of the photopolymeric layer by an appropriate solvent.
U.S. Pat. No. 4,609,252 teaches one method of lithographically forming optical elements using an acrylic photoreactive composition which is capable of forming a waveguide material upon polymerization. This patent teaches one to utilize polymers with as high a glass transition temperature as possible, i.e., 90.degree. C.-220.degree. C., in order to provide for the greatest operating temperatures. U.S. Pat. No. 5,136,682 teaches the production of waveguides using photopolymerizable compositions such as acrylics having a glass transition point, T.sub.g, of at least 100.degree. C. The foregoing waveguides, however, suffer from undesirably high optical loss and are not sufficiently flexible.
Among the many known photopolymers, acrylate materials have been widely studied as waveguide materials because of their optical clarity, low birefringence and ready availability of a wide range of monomers. However, the performance of optical devices made from many acrylate materials has been poor, due to high optical losses, poor resistance to aging and yellowing, and thermal instability of the polymerized material.
There continues to be a need for low loss radiation curable materials that can be used to make optical devices by a more direct process having fewer manufacturing steps. Specifically, a process is desired that does not require a reactive ion etching (RIE) step to develop the pattern of the optical element core. Such materials could be used to make optical devices by a relatively simple and more direct lithographic procedure.
It is also important that these materials have little or no birefringence. As is well known in this art, birefringence is the difference between the refractive index of the transverse electric or TE polarization (parallel to the substrate surface) and the transverse magnetic or TM polarization (perpendicular to the substrate surface). Such birefringence is undesirable in that it can lead to devices with large polarization dependant losses and increased bit error rates in telecommunication systems.
Another tytpe of useful optical device is a waveguide grating. Diffraction gratings, e.g., Bragg gratings, are used in the telecommunications industry to isolate a narrow band of wavelengths from a broader telecommunications signal. Polymeric planar waveguide gratings have a number of advantages in terms of their relative ease of manufacture and their ability to be tuned over a wide range of frequencies by temperature or induced stress. In addition, such devices have the advantage of being easily incorporated into integrated devices. Unfortunately, such gratings in polymeric materials typically are of relatively low efficiency. This drawback can result in poor signals with increased bit error rates. It would, therefore, be beneficial to find a method of making polymeric planar waveguide gratings with improved efficiency.
Dense Wavelength Division Multiplexing (DWDM) systems have recently attracted a lot of interest because they address the need for increased bandwidth in telecommunication networks. The use of DWDM systems allows the already installed point-to-point networks to greatly multiply their capacity without the expensive installation of additional optical fiber. DWDM systems can send multiple wavelengths (signals) over the same fiber by using passive optical components to multiplex the signals on the one end of the line and demultiplex them on the other end of the line. Polymeric materials provide a low-cost, alternative solution to a variety of optical components for DWDM.
WDM devices can be designed by using planar waveguides with gratings that can reflect a single wavelength or channel as a building block. These devices can be fabricated with low temperature processes and high throughput. In this disclosure, we focus on the properties of this fundamental building block, the fabrication of a grating in a waveguide structure, outline what we believe is the basic mechanism responsible for the grating formation, and its environmental, humidity and temperature performance.
Prior approaches to meeting these needs have not been completely satisfactory, and the present invention provides significant and unexpected improvements applicable to this technology in order to satisfy the materials, process, and device application requirements noted above.